Solid state light sources, such as LEDs, have been established as an energy efficient alternative to conventional light sources. In some high power applications, e.g. spot lights and image projection, it has been proposed to couple light from several light sources (e.g. LEDs) into one common wave length converter referred to as a concentrator. Light of a longer wavelength can then be extracted from just one surface of the concentrator, which increases the intensity of the light emitted, whereas the etendue of the light source can remain small. Document U.S. Pat. No. 7,982,229 describes a conversion structure comprising luminescent concentrator which receives light from blue LEDs, converts the light into longer-wavelength light and guides it to an exit surface, where the resulting brightness can be high.
A problem with such luminescent concentrators is effective out-coupling. If the refractive index of the converter is n=1.8, the critical angle for total internal reflection (TIR) with respect to air is 34 degrees, so that light with this or larger angle of incidence with respect to the normal to the side surfaces will be guided by TIR to the exit surface. However, if the exit surface is perpendicular to the side surfaces, much of this light (in fact, all light with an angle of incidence at the side surface smaller than 56 degrees) will have an angle of incidence at the exit surface greater than 34 degrees and will be reflected again at the exit surface. This means that light with incidence angles at the side surfaces between 34 degrees and 56 degrees will be reflected eternally in a perfect converter.
A well-known extraction structure is the compound parabolic concentrator (CPC), which can extract nearly all light. A disadvantage of such a device is that it is cumbersome to attach it to the luminescent concentrator rod and mechanically instable. Another issue when using a CPC is that the étendue is enlarged by a factor n2 (where n is the refractive index of the CPC material). This means that, for a given étendue of the application (e.g. projector), the rod should be made smaller, which in turn means that it is more difficult to accommodate enough LEDs at the sides and leads to additional bottlenecks such as thermal limits due to the reduced cooling area.
As shown in U.S. Pat. No. 7,982,229, microscopic structures like lenses or pyramids or a rough structure on the exit surface will extract more light than an unstructured surface. Such structures will reflect light that cannot exit into directions that might give rise to escape after going round by reflection to the other surfaces. However, still a large amount of light (up to 40%) will not escape from the converter.